Corrective tempering method for rolling elements

ABSTRACT

In a corrective tempering method and apparatus for a rolling element in which the thermal deformation of the rolling element is corrected within an extremely short time by utilizing plasticity exhibited during a metallic structure transforming process brought about by low-temperature tempering of the rolling element made of steel, the hardened rolling element is heated and pressured to a maximum temperature within a range of from 250 to 500° C. by a heating device while set in correcting molds to correct the deformation of the rolling element at a degree of working within a range not exceeding an elastic deformation range of the rolling element at a room temperature. The rolling element can be heated also by an induction heating device in addition to a conduction heating device. A product whose inner/outer diameter correcting degree of deformation is 60% or more, dimension standardizing rate is 30% or more, surface roughness is less than Ra 0.2 μm, and surface hardness is HRC 56 or more can be obtained for a correction time within 6 minutes and an induction heating time within 30 seconds.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement of a correctivetempering method and apparatus for correcting the hardening deformationof rolling elements.

In the process of manufacturing rolling elements such as the inner andouter races, cages, and other parts of, e.g., rolling bearings, thedeformation of the rolling elements induced by such a heat treatment ashardening has heretofore been corrected by tempering. Conventionaltempering methods include so-called "bracing tempering" and presstempering. The former method is characterized as correcting thedeformation of a rolling element by subjecting the rolling element tosuch a normally tempering treatment as keeping the rolling element inthe tempering furnace at a required temperature for 60 to 120 minuteswith the rolling element being given a reversely bracing deformationusing an appropriate tool. On the other hand, the press tempering methodis characterized as correcting the deformation of a rolling element bysubjecting the rolling element to the aforementioned normally temperingtreatment with the rolling element pressured by molds or the like beforethe tempering treatment.

However, the conventional tempering methods address several problems.One problem is that the treatment time required for correcting thedeformation of a rolling element is very long, not only because thesemethods involve the step of preparing and setting tools, molds, and thelike in consideration of the size and amount of deformation of therolling element to be corrected and pressuring the rolling element withthese tools and molds, but also because the rolling element isthereafter subjected to the normally tempering treatment for 60 to 120minutes in the furnace. More specifically, the following problems havebeen encountered.

(1) A number of tools and molds must be involved to treat a number ofrolling elements on a mass production basis. Therefore, the conventionalarts that are of low productivity cannot respond to the needs for massproduction.

(2) Further, the tempering furnace, being heated with hot blast, takes20 minutes or more for heating a rolling element, and there is a limitin shortening the treatment time imposed by this slow heating of therolling element.

(3) Still further, if atmosphere temperature is increased to shorten thetreatment time, the hardness of the rolling element is reduced.

(4) Still further, the heat-treated rolling element still holdsinconsistencies caused in turning before the heat treatment, which inturn requires that grinding margins after the heat treatment beincreased. As a result, grinding cost reduction is restricted.

SUMMARY OF THE INVENTION

The present invention has been made to overcome conventional problems.Therefore, the object of the invention is to provide a correctivetempering method and apparatus for rolling elements that can correct theheat-treatment deformation of the rolling elements within an extremelyshort time by utilizing plasticity exhibited during the metallicstructure transforming process brought about by low-temperaturetempering of the rolling elements that are made of steel.

To achieve the above object, a method according to the invention isachieved by a method of correctively tempering a rolling element of oneof a hardened cylindrical element and a hardened annular element, themethod including the steps of: inserting the rolling element into a moldto work at least one surface of an inner diameter surface and an outerdiameter surface of the rolling element; and heating the rolling elementfor a heating time within six minutes to a maximum rolling elementtemperature within 250 to 500° C. to correct a hardening deformation ofthe rolling element.

Further, an apparatus according to the invention is achieved by anapparatus for correctively tempering a rolling element, which provides:an induction heater heating the rolling element to a desired temperatureand including a heating coil; a mold constraining at least one of aninner surface, an outer surface of the rolling element and both of theinner and outer surfaces, the rolling element being at least one of ahardened cylindrical element and a hardened annular element; and a moldtemperature adjusting device arranged relative to the heating coil foradjusting a temperature of the mold, the mold temperature adjustingdevice being positioned outside the heating coil in at least onedirection of a radial direction and an axial direction of the heatingcoil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a first embodiment illustrative of a modeof heating and pressuring method of the invention;

FIG. 2 is a diagram illustrating a relationship among producttemperature, correcting degree of deformation, and pressuring andheating time in a corrective tempering method, which is the firstembodiment of the invention;

FIG. 3 is a graph illustrating a relationship among product temperature,rolling element hardness, and pressuring and heating time in the firstembodiment of the invention;

FIG. 4 is a sectional view of a second embodiment illustrative of a modeof heating and pressuring method of the invention;

FIG. 5 is a sectional view of a third embodiment illustrative of a modeof heating and pressuring method of the invention;

FIG. 6 is a sectional view of a fourth embodiment illustrative of a modeof heating and pressuring method of the invention;

FIG. 7 is a sectional view of a fifth embodiment illustrative of a modeof heating and pressuring method of the invention;

FIGS. 8A and 8B are diagrams illustrative of a method of measuring theamount of warpage of rolling elements; FIG. 8A is a sectional view inthe case of a doughnut like rolling element; and FIG. 8B is a sectionalview in the case of an annular rolling element;

FIG. 9 is a sectional view of a first embodiment of a correctivetempering apparatus according to the invention;

FIG. 10 is a sectional view showing a main portion in a secondembodiment of the corrective tempering apparatus according to theinvention;

FIG. 11 is a sectional view showing a main portion in a third embodimentof the corrective tempering apparatus;

FIG. 12 is a sectional view showing a main portion in a fourthembodiment of the corrective tempering apparatus;

FIGS. 13A and 13B each is a sectional view showing a main portion in afifth embodiment of the corrective tempering apparatus; FIG. 13A showsthe main portion before heating; FIG. 13B shows the main portion afterheating;

FIG. 14 is a sectional view showing a main portion in a sixth embodimentof the corrective tempering apparatus;

FIGS. 15A and 15B each is a sectional view showing a main portion in aseventh embodiment of the corrective tempering apparatus; FIG. 15A showsthe main portion before heating; FIG. 15B shows the main portion afterheating;

FIG. 16 is a sectional view showing a main portion in an eighthembodiment of the corrective tempering apparatus;

FIG. 17 is a diagram showing a relationship among maximum workpiecetemperatures, correcting degrees of deformation, and workpiece surfacehardnesses obtained by a corrective tempering test;

FIG. 18 is a diagram showing a relationship between maximum workpiecetemperatures and dimension standardizing rates obtained by thecorrective tempering test;

FIG. 19 is a diagram showing a relationship between degrees of workingand surface roughnesses Ra (μm) of surfaces (the outer diameter surfacein this case) obtained by the corrective tempering test, the surfacebeing corrected by a correcting mold;

FIG. 20 is a diagram showing a relationship between degrees of workingand correcting degrees of deformation obtained by the correctivetempering test;

FIG. 21 is a diagram showing an appropriate range (shaded portion) ofcorrective tempering conditions of the invention obtained from therelationship between maximum workpiece temperatures and degrees ofworking, the appropriate range being obtained by the correctivetempering test;

FIG. 22 is a diagram showing effects of the shape of a workpiece exertedon the corrective tempering conditions, the effects being obtained by acorrective tempering test;

FIG. 23A is a plan view of a correcting mold used in another deformationcorrecting test with outer races of ball bearings used as workpieces;FIG. 23B is a sectional view taken along an arrow XXIII--XXIII shown inFIG. 23A; and

FIG. 24 is a diagram showing a relationship between correcting energyand product temperature with a S/V ratio as a parameter, therelationship being obtained by the deformation correcting test carriedout using the mold shown in FIGS. 23A and 23B.

DETAILED DESCRIPTION OF THE INVENTION

Details of the present invention will hereinafter be described.

The inventors have positively made a study to overcome problemsaddressed by the conventional corrective tempering methods andapparatuses. As a result of the study, it was found out that correctivetempering can be implemented within an extremely short time (in theolder of seconds) by (1) pressuring a rolling element to be correctedusing a mold that has been heated to a predetermined temperature by aconduction heating device, and heating and pressuring the rollingelement while transmitting the heat of the mold to the rolling elementby conduction or by (2) heating and pressuring a rolling element to becorrected while pressuring the rolling element by a mold with therolling element being heated to a predetermined temperature directly byan electromagnetic induction heating device. Hence, the inventors havefound out that the deformation of a rolling element can be correctedwithin a short time with a smaller reduction in hardness compared withthe corrective tempering methods using a conventional furnace.

That is, the invention is characterized in that carbon dissolved inmartensite immediately after the hardening of a rolling element made ofsteel is precipitated and diffused as ε-carbides by heating the rollingelement to 250 to 500° C. and that toughness is imparted to the rollingelement by transforming the martensite from the tetragonal to the cubic.And post-treatment deformation of the rolling element such as warpage aswell as pre-treatment mechanically produced dimensional inconsistenciesare corrected by utilizing plasticity exhibited during this process.

Thus, in the corrective tempering method for rolling elements accordingto the invention, the amount of deformation such as warpage, dimensions,and the like can be controlled within a prescribed value and, at thesame time, hardness is kept so as not to be reduced below HRC 56 whichis required for the rolling element in order to maintain the rollingfatigue life.

Steel materials of which rolling elements of the invention are made are:carbon steels containing 0.5 wt % or more of carbon, bearing steelscontaining about 1% of carbon, or case hardening steels containing 0.5wt % or less of carbon before case hardening and 0.6 wt % or more ofcarbon in the case after case hardening. These steel materials are usedeither singly or in combination.

The reasons why the deformation correcting temperatures are set tovalues between 250 and 500° C. are as follows.

In the case of using bearing steels and case hardening steels inparticular, deformation correction within a short time (within 6 minuteswhen mass production of the rolling elements is considered) is hard ifthe mold temperature is less than 250° C. On the other hand, at a moldtemperature exceeding 500° C. in the case of conduction heating, notonly hardness is reduced from HRC 56 that is required for bearingsalthough the amount of deformation corrected remains unchanged, but alsothe durability of the mold is impaired. Therefore, mold temperaturesranging from 250 to 500° C. are desirable from the viewpoint of thecontrol of product heating temperature. However, even if the moldtemperature exceeds 500° C. (e.g., also 700° C.), such mold temperatureis allowed as long as a rolling element is tempered at a maximum railingelement temperature within a range of from 250 to 500° C. for a time notexceeding 6 minutes.

Further, a corrective tempering time not exceeding 6 minutes is desiredbecause the heating time must match the hardening speed of a rollingelement during the manufacturing process.

The corrective tempering methods are characterized in that: after acylindrical or annular rolling element has been subjected to hardening,the thus hardened rolling element is inserted into a mold whosetemperature has already been adjusted; and the rolling element is thensubjected to tempering by heating using a heating device to a hightemperature within a range of from 250 to 500° C. for a short time withat least one of the outer surfaces (the outer diameter surface, theinner diameter surface, both end faces) of the rolling element bound bythe mold. Conduction heating or induction heating is employed as theheating device.

In the case of induction heating, the heating time is within 30 seconds,which is thus advantageous in quickening the tempering treatment. Inthis case, the mold is preferred to be made of a material less affectedby high frequencies. A material whose magnetic permeability is 1.5 orless is desirable; e.g., ceramic is desirable. The reason therefor isthat corrective tempering can be implemented precisely, quickly, as wellas reliably because such a material can prevent the mold temperaturefrom increasing due to lines of magnetic force produced by the inductionheating coil, so that the mold temperature can be controlledindependently of the induction heating source.

Further, the mold must have a required strength. Materials other thanceramic capable of meeting these requirements are: nonmagnetic steels,aluminum, brass, and copper.

The expansion coefficient of a ceramic mold is about 1/5 that of a steelmold. Therefore, a rolling element within the induction-heated mold isthermally expanded to be compressed and corrected strongly by theceramic mold. At the same time, the rolling element precipitates anddiffuses ε-carbides from the martensite at the time of hardening due tothe effect of tempering, so that tempering is completed with thestructural transformation.

According to the corrective tempering methods for rolling elementsaccording to the invention, the deformation of a rolling element inducedby hardening can be corrected by utilizing plasticity exhibited duringthe process of tempering the hard steel structure produced in thehardened rolling element. That is, the deformation can be correctedwithin an extremely short time. In addition, a minimum hardness of HRC56 required to maintain the rolling fatigue life of a rolling elementcan be ensured, and the amount of warpage or deformation in the outer orinner diameter required to guarantee product accuracies can becontrolled under a prescribed value. Moreover, a number of rollingelements can be manufactured on a mass production basis with a smallnumber of tools and molds.

Still further, the corrective tempering methods of the invention canstandardize the dimensions of rolling elements, which in turncontributes to improving roundness and inclination. Still further, thesurface roughness of a rolling element can be improved by increasing themargin of fit between the rolling element and the mold. Morespecifically, such requirements as a correcting degree of deformation of60% or more, a surface hardness of HRC 56 or more, a corrected surfaceroughness of Ra 0.2 μm or less, and a dimension standardizing rate of30% or more can be achieved by the invention.

The corrective tempering apparatus of the invention is characterized inthat: the induction heater is employed as a source for heating a rollingelement, and that the mold temperature adjusting device is arrangedoutside the heating coil of the induction heater, so that the moldtemperature can be controlled independently of the induction heatingsource, which in turn achieves precise, quick, and reliable correctivetempering of the rolling element.

The temperature adjuster, which is the mold temperature adjusting deviceof the corrective tempering apparatus of the invention, is constructedof a temperature control panel with a temperature sensor and atemperature adjusting source embedded therein. The temperature adjustingsource is formed by either one of an electric heater or a heat mediumsolution circulation path alone or in combination with a cooling devicesuch as a cooling water circulation path. The temperature adjustingpanel is made, e.g., of a steel plate. The temperature adjusting panelmay also be made of a metal that exhibits good heat conductivityincluding ceramic such as SiC, copper, and aluminum. This temperaturecontrol panel is arranged so as to be in intimate contact with thecorrecting mold, and the temperature of the mold is adjusted to arequired temperature by the temperature sensor and the temperatureadjusting source. If the correcting mold is made of ceramic, such moldis not heated by high frequencies from the induction heating sourcebecause the magnetic permeability of the mold is 1.5 or less. Therefore,the mold can be kept precisely at a required temperature only byadjusting the mold temperature adjusting device to a predeterminedtemperature.

Precise, quick, and reliable corrective tempering of a rolling elementcan be implemented by the corrective tempering apparatus of theinvention while utilizing this function of the mold temperatureadjusting device. That is,

(1) If the mold temperature adjusting device is set to a temperaturethat will adjust the mold temperature in such a manner a rollingelement, which is a workpiece heated by the induction heater, is notthermally expanded to be in intimate contact with the mold, theworkpiece can be set in the mold quickly under low forcing pressure.

Further, the induction-heated workpiece is cooled through conduction ofthe heat thereof to the ceramic mold whose temperature has been adjustedby the mold temperature adjusting device, which in turn allows theworkpiece to be dropped naturally from the mold due to thermalshrinkage. As a result, the workpiece is no longer required to beforcibly extracted out of the mold with an extracting press. Hence, thetemperature of the ceramic mold is controlled independently by the moldtemperature adjusting device, which in turn allows the operation ofsetting the workpiece in and taking the workpiece out of the mold to becompleted within an extremely short time. This is a remarkable advantagefor a continuous process.

(2) In the corrective tempering apparatus of the invention, a ratio ofvolume between a workpiece to be treated and a ceramic mold is set to 1to 100 or so from the viewpoint of strength. However, when heattreatment is performed only by induction heating without employing themold temperature adjusting device, induction heating efficiency isreduced because the heat conduction from the induction-heated workpieceto the non-induction-heated ceramic mold is large. Further, aninconsistent temperature distribution between a workpiece portion thatis in contact with the mold and a workpiece portion remote from the moldbrings about a problem of inconsistent product quality. If, on the otherhand, the temperature of the ceramic mold is controlled independently bythe mold temperature adjusting device, efficient and uniform inductionheating can be implemented. Thick-walled annular bodies, in particular,are more susceptible to tempering quality inconsistency with largerdifference between the mold temperature and the workpiece temperature atthe time of heating. In such a case, tempering quality inconsistency canbe suppressed as small as possible by heating the rolling element mainlythrough conduction heating from the mold with the mold temperature beingkept at a high temperature by the mold temperature adjusting device.That is, by adjusting the ratio of induction heating to conductionheating, heat treatment conditions optimal to the shape of the workpieceand the treatment tact can be selected.

Next, preferred embodiments of the invention will be described.

In a first embodiment of the invention shown in FIG. 1, a rollingelement 1A has a flat doughnut like shape with an outer diameter of 40mm, an inner diameter of 18 mm, and a thickness of 1.2 mm, and is madeof a high carbon bearing steel (SUJ2 according to Japanese IndustrialStandard) containing 0.95 to 1.10% of carbon. The hardness of therolling element 1A after hardening is HRC 66. The rolling element 1Athat has been hardened was subjected to a warpage correcting treatmentto correct the warpage produced during the hardening. The warpagecorrecting treatment was carried out by interposing the rolling element1A between heated upper and lower molds 2, 3 and holding the element 1Afor a predetermined time with a pressure P equal to 1 ton applied fromboth top and bottom.

FIG. 2 shows deformation correcting temperature, i.e., maximumtemperature reached by the rolling element 1A (hereinafter referred toas the "product temperature") in function of the amount of warpage ofthe rolling element 1A with heating and pressuring time as a parameter.That is, FIG. 2 shows the results of measurements made when the rollingelement 1A made of a bearing steel was subjected to a correctivetempering treatment by pressure. The initial amount of warpage of therolling element 1A before the treatment was 0.43±0.09 mm. As is apparentfrom FIG. 2, a satisfactory deformation correction was made by heatingthe product to 250° C. or more. That is, by heating the product to 250°C. or more, a warpage requirement of the product of 0.2 mm or less (themaximum allowable warpage) was met for a heating and pressuring time notexceeding 6 minutes.

FIG. 3 shows product temperature in function of rolling element 1Ahardness with the heating and pressuring time as a parameter in theaforementioned case. As is apparent from FIG. 3, the higher the moldtemperature and the longer the heating and pressuring time, the lowerthe hardness. Therefore, a satisfactory deformation correction can bemade within a range A defined in FIG. 3. The range A ensures for therolling element a hardness of HRC 56 or more with pressure being appliedfor such a short time as not exceeding 6 minutes.

FIG. 4 shows a second embodiment of the invention. An annular rollingelement 1B (e.g., the outer race of a rolling bearing) is heated andpressured by applying a pressure P from sideways (orthogonal to the axisof the rolling element) using a correcting split mold 5 that has alreadybeen heated to a predetermined temperature. In this case, thedeformation of the outer diameter is corrected by binding the outerdiameter surface of the rolling element 1B. It may be noted thatdeformation correction using the split mold can be applied also to thedoughnutlike rolling element 1A, which is the first embodiment.

FIG. 5 shows a third embodiment of the invention. The annular rollingelement 1B is forced into a hole 7 of a die 6 to correct deformation.The die 6 has predetermined dimensions and serves as a correcting mold.The rolling element 1B has the deformation of the outer diameter thereofcorrected with a pressure applied from sideways while passing though thetapered portion 8 of the already heated hole 7.

FIG. 6 shows a fourth embodiment of the invention. The outer race (orthe inner race) of a ball bearing 1C, which is a rolling element, hasthe deformation thereof produced by hardening corrected. In this case,the deformation is corrected by not only applying a pressure P to bothend faces of the outer race of the ball bearing 1C with already heatedcylindrical pressing molds 10, 10, but also pushing an already heatedmold 11, which is a tapered shaft or a straight shaft, into the centerbore of the outer race of the ball bearing 1C. As a result of thisoperation, the warpage on the end faces of the outer race of the ballbearing 1C as well as the deformation of the inner diameter thereof canbe corrected simultaneously.

Further, if the warpage on the end faces is not so serious, only onemold 10 may be used in order to push the outer race of the ball bearing1C so as to allow the already heated mold 11 to be inserted into theouter race, instead of causing both sides of the outer race to bepressured by the already heated pressing molds 10, 10. In this case, theother pressing mold 10 is abutted against one end face of the outer raceof the ball bearing 1C through, e.g., a spring and is used only forheating the outer race of the ball bearing 1C. Thus, the other pressingmold 10 may not necessarily be required to be used.

FIG. 7 shows a fifth embodiment of the invention. The deformation of theouter diameter of the rolling element 1B is corrected by inserting therolling element 1B into a pressuring mold 16 made of ceramic (e.g.,alumina, SiC, Si₃ N₄, or diamond), thereafter heating the rollingelement 1B using an inductor coil 19 arranged outside the pressuringmold 16, so that the deformation of the outer diameter can be correctedby the fitting force as well as the pressuring force produced betweenthe rolling element 1B and the pressuring mold 16 by the thermalexpansion of the rolling element 1B.

In either one of the aforementioned embodiments, not only the amount ofdeformation such as warpage can be controlled within a prescribed valuecorresponding to the size of a rolling element, but also hardness can bemaintained at HRC 56 or more by a quick treatment.

It may be noted that an exemplary method of measuring the amount ofwarpage is shown in FIGS. 8A and 8B.

FIG. 8A shows a case where the amount of deformation of the doughnutlikerolling element 1A is measured. The amount of vertical movement of oneend of the rolling element 1A is measured with a dial gauge 14 byplacing the rolling element 1A on a surface plate 12 and biasing theopposite end of the rolling element 1A intermittently with a finger tipwith the outer diameter surface of the rolling element 1A held by alocating ring 13 to prevent horizontal displacement.

FIG. 8B shows a case where the amount of deformation of the annularrolling element 1B is measured. The amount of deformation of the outerdiameter is measured with the dial gauge 14 by rotating the rollingelement 1B in such a manner as shown in FIG. 8B while placing therolling element 1B on the surface plate 12, bringing one end of theouter diameter surface of the rolling element 1B into contact with a Vblock 15 fixed on the surface plate 12 at two points, and bringing theouter diameter surface on the other end into contact with the dial gauge14 at one point.

It may be noted that the method of heating the heating and pressuringmold in each of the aforementioned embodiments is not particularlylimited. A method in which a mold whose heat capacity is large is heatedin a furnace in advance, a method in which a mold incorporating aheating source therein is heated by the heating source, an inductionheating method, or a like method may be selected arbitrarily.

Still further, it is required that a mold already heated to apredetermined temperature be kept heated for a predetermined time(within 6 minutes). Therefore, the heated mold is placed in a heatedatmosphere so that correction by pressure may be effected in such heatedatmosphere.

Still further, correction by pressure may also be effected by using apressuring mold made of ceramic and heating the rolling element throughinduction heating from outside such pressuring mold.

Next, corrective tempering apparatuses according to the invention willbe described.

FIG. 9 is a first embodiment of the corrective tempering apparatusaccording to the invention. A cylindrical ceramic mold 22 whose magneticpermeability is 1.5 or less is set in a main body 21 of a correctivetempering apparatus 20. A workpiece W, which is an annular rollingelement, is inserted into the inner diameter surface of the mold 22 withthe outer diameter surface thereof bound. Induction heating coils 23serving as workpiece heating device are arranged on both upper and lowerend faces of the mold 22 in proximity. In this case, the mold 22 is madeas thin as possible so that the induction heating coils 23 can bearranged as close to the workpiece W to be heated as possible. Inaddition, each coil has two turns so that the workpiece W can be heateduniformly from both ends thereof by induction heating. The innerdiameter of each coil 23 is made slightly larger than the inner diameterof the mold 22.

A mold temperature adjuster 24 serving as a mold temperature adjustingdevice for adjusting the temperature of the mold 22 is arranged outsideone of the heating coils 23, which is the induction heating source, inthe radial direction of the coil 23 so as to be in intimate contact withthe upper end face of the ceramic mold 22. The mold temperature adjuster24 of this embodiment is formed into a flat doughnut like shape. Themold temperature adjuster 24 includes a steel (SC) panel 25, atemperature adjusting source 26, and a not shown temperature sensor 27.The steel panel 25 is used as a conducting member for conducting heatsatisfactorily. The temperature adjusting source 26 such as an electricheater or a heat medium solution and the temperature sensor 27 such as athermocouple are incorporated in the mold temperature adjuster. Theupper surface of the mold temperature adjuster is covered with aninsulating material 28. The flat surface of the steel panel 25 isarranged so as to be in intimate contact with the flat surface of themold 22.

Up to a mold temperature of about 300° C., the mold temperature adjuster24 is of such type that a heat medium oil is circulated in a coil-likegroove formed inside the mold temperature adjuster 24 or that anelectric heater is embedded inside the mold temperature adjuster 24. Ifthe mold temperature exceeds 300° C., it is preferred that the moldtemperature adjuster 24 be of the latter type (electric heater type),because the heat medium oil type addresses a problem of heatdeterioration. If cooling is required to keep mold temperature constant,water or oil is circulated in the coil-like groove formed in the moldtemperature adjuster 24. Temperature control is effected by making achangeover between the heating source and the cooling source based on adetection signal from the temperature sensor 27 embedded in the moldtemperature adjuster 24.

Although close to the induction heating coils 23, the mold 22 is made ofceramic whose magnetic permeability is 1.5 or less and the mold 22 willnot therefore be further heated by the lines of magnetic force producedby the induction heating coils 23. Hence, the mold 22 temperature can beadjusted by the mold temperature adjuster 24 alone.

Not only a workpiece forcing tool 30 and a main cylinder 31 forvertically moving the workpiece forcing tool 30 are arranged at theupper end, but also a workpiece supporting tool 32 and a sub-cylinder 33for vertically moving the workpiece supporting tool 30 are arranged atthe lower end along the center shaft of the apparatus main body 21.

Further, a workpiece w insertion port 36 and an insertion chute 37 arearranged on an upper side surface of the apparatus main body 21, whereasa workpiece discharge cylinder 38, a treated workpiece discharge port39, and a treated workpiece discharge chute 40 are arranged on a lowerside surface of the apparatus main body 21.

An operation of the corrective tempering apparatus 20 will be describednext.

The workpiece forcing tool 30 and the workpiece supporting tool 32 arefirst set in the upper and lower evacuated positions, respectively.

The workpiece supporting tool 32 is elevated to the workpiece insertionport 36 on the upper side of the apparatus by operating the sub-cylinder33 on the lower side of the apparatus. Then, a workpiece W is chargedonto the workpiece supporting tool 32 from the insertion chute 37 viathe workpiece insertion port 36. It is so designed that the chargedworkpiece W is automatically aligned with the mold 22.

Once the workpiece W has been charged onto the workpiece supporting tool32, the workpiece forcing tool 30 is lowered by operating the maincylinder 31 so that the workpiece W is clamped between both tools 30,32. The tools 30, 32 are lowered with the workpiece W horizontallyclamped therebetween, pass through the upper induction heating coil 23,and are forced into the middle portion of the mold 22 from the uppertapered portion of the inner diameter surface of the ceramic mold 22.After being forced, the workpiece forcing tool 30 is elevated, whereasthe workpiece supporting tool 32 is lowered to move away from theworkpiece W. The workpiece W is subjected to induction heating byoperating the induction heating coils 23 under this condition. It ispreferred that the maximum workpiece W temperature range from 250 to500° C. and that the induction heating time for keeping the workpiece atsuch maximum temperature not exceed 30 seconds. Even if the workpiece Wis heated for more than 30 seconds, the effect of tempering anddeformation correcting capability will not be improved. That is, heatingfor a time exceeding 30 seconds will be wasted.

Upon end of the heating, the workpiece forcing tool 30 is lowered to alarge extent to push the workpiece W out toward the lower portion of themold 22 by operating the main cylinder 31, and the workpiece W isreceived by the elevated workpiece supporting tool 32. Then, theworkpiece supporting tool 32 is lowered so that the workpiece W islocated at the workpiece discharge port 39. The workpiece W on theworkpiece supporting tool 32 is thereafter thrown out onto the workpiecedischarge port 39 and sent to the discharge chute 40 by operating theworkpiece discharge cylinder 38. As a result of these steps, one cycleof the operation is completed.

If induction heating efficiency is improved, or if a small-capacity highfrequency power source is used for the workpiece, it is suggested thatinduction heating be effected by interposing highly permeable membersbetween the workpiece forcing tool 30 and a corresponding contactsurface of the workpiece W and between the workpiece supporting tool 32and the corresponding other contact surface of the workpiece W with theworkpiece interposed between both tools. By first induction-heating thehighly permeable members to a high temperature and then by utilizing theheat conduction effect of the induction heating, the workpiece W isheated with high efficiency.

Further, according to the corrective tempering apparatus 20, theworkpiece W can also be subjected to an ironing process inside theceramic mold 22 during the heating in order to improve the surfaceroughness of the outer diameter surface of the workpiece W.

FIGS. 10 to 16 show various embodiments of the invention, which aremodifications of the apparatus, and modes of arrangement of thecorrecting mold 22, the induction heating coil 23, and the moldtemperature adjuster 24, in particular. It may be noted that the same orlike parts and components are denoted as the same reference numerals.

FIG. 10 shows a second embodiment of the corrective tempering apparatus.The outer diameter of the workpiece W is corrected by induction heatingbased on an inner diameter heating method. The mold 22 is arrangedoutside the induction heating coil 23 in the radial direction of thecoil. The mold temperature adjuster 24 is arranged so as to be inintimate contact with the outer diameter surface of the mold 22.

FIG. 11 shows a third embodiment of the corrective tempering apparatus.The outer diameter of the workpiece W is corrected by induction heatingbased on an outer diameter heating method. The induction heating coil 23is arranged outside the mold 22 in the radial direction of the mold, themold 22 having the workpiece W forced thereinto. The mold temperatureadjusters 24 are arranged so as to be in intimate contact with the mold22 and so as to be distant from the coil 23 in the axial direction ofthe coil so that the mold temperature adjusters 24 are less affected byhigh frequencies.

FIG. 12 shows a fourth embodiment of the corrective tempering apparatus.The inner diameter of the workpiece W is corrected. A collet typecorrecting mold 22 is employed. The inner diameter of the workpiece W iscorrected by expansion while inserting the mold 22 into the innerdiameter surface of the workpiece W and forcing a collet into the mold22. The induction heating coil 23 is arranged outside the workpiece W.The mold temperature adjusters 24 are arranged outside the mold 22 inthe axial direction of the coil.

FIGS. 13A and 13B show a fifth embodiment of the corrective temperingapparatus. The inner diameters of the workpieces W are corrected. FIG.13A shows the molds 22 which is made of a material whose linearexpansion coefficient is larger than those of the workpieces W. FIG. 13Bshows the mold 22 which is heated by the induction heating coil 23arranged outside the outer diameter surface of each workpiece W isthermally expanded to come in intimate contact with the inner diameterof the workpiece, so that the inner diameter of each workpiece is heatedand pressured.

FIG. 14 shows a sixth embodiment of the corrective tempering apparatus.Both the inner and outer diameter surfaces of the workpiece W arecorrected. This is an example in which the inner and outer diametersurfaces are corrected simultaneously using a collet mold 22 A for theinner diameter surface and a cylindrical mold 22B for the outer diametersurface. The induction heating coils 23 are arranged on both upper andlower end faces of both molds. The mold temperature adjuster 24 isarranged outside the coils 23 in the radial direction of the coils 23.

FIGS. 15A and 15B each shows a seventh embodiment of the correctivetempering apparatus. The deformation of the workpiece W is correctedtaperingly. This is an example in which not only the deformation iscorrected by tapering, but also the outer and inner races of conicalbearings are fabricated. The inner diameter surface of the mold 22 istapered. A pressing tool 41 is employed, the tool 41 pressuring theworkpiece W inserted into the mold 22 in the axial direction. Theinduction heating coils 23 are arranged on both upper and lower endfaces of the tapered mold 22. The mold temperature adjuster 24 isarranged outside the coils 23 in the radial direction of the coils.

FIG. 16 shows an eighth embodiment of the corrective temperingapparatus. A plurality of workpieces W are processed at once using onlyone mold. The mold 22 is a vertically elongated cylindrical body. Theworkpieces W are inserted inside the mold 22 while placed one uponanother in the axial direction of the mold. The outer diameter surfacesof the thus inserted workpieces are subjected to a continuous correctivetempering treatment. The outer diameter surfaces of the workpieces arebound by heating to be corrected by tempering and are thereafterseparated from the mold by utilizing the shrinkage of the workpieces Wbrought about by cooling. The induction heating coils 23 are arranged ina plurality of rows so as to enclose the outer diameter surface of thecylindrical mold 22. The mold temperature adjusters 24 are arrangedoutside these coils 23 in the axial direction of the coils.

While the mold temperature adjusters are arranged so as to neighbor thecorrecting mold in the aforementioned apparatus and their modifications,the mold temperature adjusters may also be arranged inside the mold.

A comparative corrective tempering test carried out using theaforementioned corrective tempering apparatuses 20 will be described.

Thin-walled annular rolling elements made of steel were used asworkpieces W. The steel grades selected were SUJ2, SCR420, SUS440C.These workpieces W were heated to 840° C. and kept at such temperaturefor 30 minutes. Then, the workpieces W were subjected to hardening usinga hardening oil at 60° C. and were hardened to obtain specimens.

A total of five corrective tempering ceramic molds 22 were used. Theirinner diameters were: 47.000 mm, 50.150 mm, 65.150 mm, 61.700 mm, and62.200 mm.

A total of four mold temperatures were set by the mold temperatureadjusters 24. The four mold temperatures were: 55° C., 200° C., 400° C.,and 500° C.

A total of six maximum workpiece W temperatures were set by theinduction heating coil 23. The six maximum workpiece W temperatureswere: 200° C., 250° C., 300° C., 400° C., 500° C., and 600° C.

Table 1 shows evaluation items, evaluation criteria, and judgmentsymbols per evaluation item for the comparative test.

                                      TABLE 1                                     __________________________________________________________________________                Standardizing                                                                        Surface                                                                             Rapid Treat-                                                                           Tempering Consistency                            Degree of                                                                            rate of                                                                              Roughness                                                                           ment Time (sec)                                                                        (ΔH.sub.v) (surface                   Evaluation                                                                         Correction (%)                                                                       dimensions (%)                                                                       (Ra)  (including set time)                                                                   hardness dispersion)                        __________________________________________________________________________    A    80≦                                                                           80≦                                                                           <0.1   0-35     0-16                                       B    60-80  50-80  0.1-0.2                                                                             35-60    16-31                                                          (inclusive)                                                C    --     30-50  --     60-360  --                                          D    <60    <30    not changed                                                                         360≦                                                                            31≦                                  __________________________________________________________________________     (Note)                                                                        Evaluation is ranked with the following degrees:                              A: very superior                                                              B: superior                                                                   C: usable                                                                     D: notusable.                                                            

The correcting degree of deformation (degree of correction) in theinner/outer diameter of a workpiece W was calculated in accordance withthe following equation. ##EQU1##

Further, the dimension standardizing rate was calculated in accordancewith the following equation. ##EQU2##

The object of the corrective tempering according to the invention isthat the treated workpieces W satisfy the following conditions.

The correcting degree of the deformation of the inner/outer diameter is60% or more.

The rate of standardizing the dimension of the inner/outer diameter is30% or more.

The surface roughness of the corrected surface is less than Ra 0.2 μm.

The rapid treatment time is within 360 seconds (6 minutes).

The surface hardness is HRC 56 or more.

Tables 2A to 2D show the test conditions and the test results.

The column for the degree of working (%) in Table 2A shows a degree ofworking δ, which is calculated in accordance with the followingequation.

    δ={D/d(1+α·T)-1}×100            (1)

where D: Outer diameter of a workpiece before corrective tempering (mm)

d: Inner diameter of a correcting mold (mm)

α: Linear expansion coefficient of a steel grade (e.g., 0.000012 forSUJ2)

T: Maximum workpiece temperature (° C.)

                                      TABLE 2A                                    __________________________________________________________________________                                Degree of                                                   Steel   Dimensions                                                                              Working                                                     Species                                                                            No.                                                                              Inner                                                                            Outer                                                                            Height                                                                            250° C.                                                                    500° C.                                                                    S  V   S/V                                __________________________________________________________________________    Inventive Examples                                                                      SUJ2 A1 43.78                                                                            46.81                                                                            7.00                                                                              --  0.20                                                                              1029                                                                             1509                                                                              0.68                                              A2 43.78                                                                            46.95                                                                            7.00                                                                              0.20                                                                              0.50                                                                              1032                                                                             1581                                                                              0.65                                              A3 43.78                                                                            47.00                                                                            7.00                                                                              0.30                                                                              0.60                                                                              1033                                                                             1606                                                                              0.64                                              A4 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              A5 43.78                                                                            47.17                                                                            7.00                                                                              0.66                                                                              0.96                                                                              1037                                                                             1694                                                                              0.61                                              A6 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SUJ2 B1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.93                                                                              1036                                                                             1685                                                                              0.62                                              B2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SCR420                                                                             C1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              C2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SUS440C                                                                            D1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              D2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SUJ2 E1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              E2 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              E3 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              E4 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                         SUJ2 F1 42.00                                                                            50.26                                                                            6.40                                                                              0.62                                                                              0.92                                                                              1010                                                                             3829                                                                              0.26                                              F2 42.00                                                                            50.26                                                                            6.40                                                                              0.62                                                                              0.92                                                                              1010                                                                             3829                                                                              0.26                                              F3 42.00                                                                            50.26                                                                            6.40                                                                              0.62                                                                              0.92                                                                              1010                                                                             3829                                                                              0.26                                              F4 42.00                                                                            50.26                                                                            6.40                                                                              0.62                                                                              0.92                                                                              1010                                                                             3829                                                                              0.26                                         SUJ2 G1 55.00                                                                            65.35                                                                            8.00                                                                              0.61                                                                              0.91                                                                              1642                                                                             7823                                                                              0.21                                              G2 55.00                                                                            65.35                                                                            8.00                                                                              0.61                                                                              0.91                                                                              1642                                                                             7823                                                                              0.21                                              G3 55.00                                                                            65.35                                                                            8.00                                                                              0.61                                                                              0.91                                                                              1642                                                                             7823                                                                              0.21                                              G4 55.00                                                                            65.35                                                                            8.00                                                                              0.61                                                                              0.91                                                                              1642                                                                             7823                                                                              0.21                                         SUJ2 H1 50.00                                                                            61.90                                                                            9.00                                                                              0.63                                                                              0.93                                                                              1749                                                                             9408                                                                              0.19                                              H2 50.00                                                                            61.90                                                                            9.00                                                                              0.63                                                                              0.93                                                                              1749                                                                             9408                                                                              0.19                                              H3 50.00                                                                            61.90                                                                            9.00                                                                              0.63                                                                              0.93                                                                              1749                                                                             9408                                                                              0.19                                         SUJ2 I1 41.50                                                                            62.40                                                                            7.50                                                                              0.62                                                                              0.92                                                                              1470                                                                             12785                                                                             0.11                                              I2 41.50                                                                            62.40                                                                            7.50                                                                              0.62                                                                              0.92                                                                              1470                                                                             12785                                                                             0.11                               Comparative Examples                                                                    SUJ2 J1 43.78                                                                            46.70                                                                            7.00                                                                              -1.34                                                                             -0.04                                                                             1026                                                                             1452                                                                              0.71                                              J2 43.78                                                                            46.72                                                                            7.00                                                                              -0.30                                                                             0.00                                                                              1027                                                                             1462                                                                              0.70                                              J3 43.78                                                                            47.23                                                                            7.00                                                                              0.79                                                                              1.09                                                                              1038                                                                             1725                                                                              0.60                                         SUJ2 K1 50.00                                                                            61.90                                                                            9.00                                                                              0.63                                                                              0.93                                                                              1749                                                                             9408                                                                              0.19                                         SUJ2 L1 41.50                                                                            62.40                                                                            7.50                                                                              0.62                                                                              0.92                                                                              1470                                                                             12785                                                                             0.11                                              L2 41.50                                                                            62.40                                                                            7.50                                                                              0.62                                                                              0.92                                                                              1470                                                                             12785                                                                             0.11                                         SUJ2 M1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              M2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SUJ2 N1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              N2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                                         SUJ2 O1 43.78                                                                            47.15                                                                            7.00                                                                              0.62                                                                              0.92                                                                              1036                                                                             1684                                                                              0.62                                              O2 43.78                                                                            47.20                                                                            7.00                                                                              0.73                                                                              1.03                                                                              1037                                                                             1710                                                                              0.61                               __________________________________________________________________________

                                      TABLE 2B                                    __________________________________________________________________________                       Mold                                                                          Dimensions                                                 Steel         Pre- (Correcting Corrective Tempering                           Species    No.                                                                              treatment                                                                          Part) Mold Temp.                                                                          Heating Temp.                                                                            Heating Time                        __________________________________________________________________________    Inventive                                                                           SUJ2 A1 Lathe                                                                              47.000                                                                               55° C.                                                                      500° C.                                                                        3 sec.                                 Examples   A2                  250° C.                                                                        3 sec.                                            A3                  300° C.                                                                    ×                                                                          10 sec.                                            A4                  400° C.                                                                       30 sec.                                            A5                  500° C.                                            A6                                                                       SUJ2 B1 CRF  47.000                                                                               55° C.                                                                      400° C.                                                                    ×                                                                           3 sec.                                            B2                  500° C.                                                                       10 sec.                                       SCR420                                                                             C1 Lathe                                                                              47.000                                                                               55° C.                                                                      400° C.                                                                    ×                                                                           3 sec.                                            C2                  500° C.                                                                    ×                                                                          10 sec.                                       SUS440C                                                                            D1 Lathe                                                                              47.000                                                                               55° C.                                                                      400° C.                                                                    ×                                                                           3 sec.                                            D2                  500° C.                                                                    ×                                                                          10 sec.                                 SUJ2       E1 Lathe                                                                              47.000                                                                               55° C.                                                                      Induction Heating                                                                         3 sec.                                        E2            200° C.                                                                      400° C.                                                                            3 sec.                                        E3            400° C.    2 sec.                                        E4            500° C.                                                                      Only Conduction Heating                                                                  20° C.                                                      400° C.                                 SUJ2       F1 Lathe                                                                              50.150                                                                               55° C.                                                                      Induction Heating                                                                        10 sec.                                        F2            200° C.                                                                      400° C.                                                                            7 sec.                                        F3            400° C.    6 sec.                                        F4            500° C.                                                                      Only Conduction Heating                                                                  30 sec.                                                            400° C.                                 SUJ2       G1 Lathe                                                                              65.150                                                                               55° C.                                                                      Induction Heating                                                                        25 sec.                                        G2            200° C.                                                                      400° C.                                                                           15 sec.                                        G3            400° C.   10 sec.                                        G4            500° C.                                                                      Only Conduction Heating                                                                  40 sec.                                                            400° C.                                 SUJ2       H1 Lathe                                                                              61.700                                                                              200° C.                                                                      Induction Heating                                                                        25 sec.                                        H2            400° C.                                                                      400° C.                                                                           20 sec.                                        H3            500° C.                                                                      Only Conduction Heating                                                                  60 sec.                                                            400° C.                                 SUJ2       I1 Lathe                                                                              62.200                                                                              400° C.                                                                      Induction Heating                                                                        27 sec.                                                            400° C.                                            I2            500° C.                                                                      Only Conduction Heating                                                                  80 sec.                                                            400° C.                                 Comparative                                                                         SUJ2 J1 Lathe                                                                              47.000                                                                               55° C.                                                                      250° C.                                                                    ×                                                                           3 sec.                                 Examples   J2                  500° C.                                                                        7 sec.                                            J3                  Impossible for Forcing                         SUJ2       K1 Lathe                                                                              61.700                                                                               55° C.                                                                      Induction Heating                                                                        30 sec.                                                            400° C.                                 SUJ2       L1 Lathe                                                                              62.200                                                                               55° C.                                                                      Induction Heating                                                                        30 sec.                                                            400° C.                                            L2            200° C.                                                                      Induction Heating                                                                        30 sec.                                                            400° C.                                 SUJ2       M1 Lathe                                                                              47.000                                                                               55° C.                                                                      200° C.                                                                    ×                                                                           3 sec.                                            M2                         30 sec.                                 SUJ2       N1 Lathe                                                                              47.000                                                                               55° C.                                                                      600° C.                                                                    ×                                                                           3 sec.                                            N2                         30 sec.                                 SUJ2       O1 Lathe                                                                              Bracing Tempering                                                     O2      200° C.    2 Hr                                     __________________________________________________________________________

                                      TABLE 2C                                    __________________________________________________________________________                  Outer Diameter                                                  Steel                          Roughness                                                                          Inner Diameter                            Species    No.                                                                              Correction Dispersion                                                                          (Ra) Correction Dispersion                     __________________________________________________________________________    Inventive                                                                           SUJ2 A1 B          A     B    B          C                              Examples   A2 250° C. Heated Element                                                            B  C  B    250° C. Heated                                                                    Clement                                                                          B                                      A3 300° C. Heated Element                                                            B  B  B    300° C. Heated                                                                    Blement                                                                          B                                      A4 400° C. Heated Element                                                            A  A  A    400° C. Heated                                                                    Blement                                                                          C                                      A5 500° C. Heated Element                                                            A  A  A    500° C. Heated                                                                    Blement                                                                          C                                      A6                  A                                              SUJ2       B1 All A                 All A                                                B2                                                                 SCR420     C1 400° C. Heated Element                                                            A  A  A    400° C. Heated                                                                    Blement                                                                          C                                      C2 500° C. Heated Element                                                            A  A  A    500° C. Heated                                                                    Blement                                                                          C                           SUS440C    D1 400° C. Heated Element                                                            A  A  A    400° C. Heated                                                                    Blement                                                                          C                                      D2 500° C. Heated Element                                                            A  A  A    500° C. Heated                                                                    Blement                                                                          C                           SUJ2       E1 A          Impossible                                                                          A    B          Impossible                                E2            due to same           due to same                               E3            dimensions            dimensions                                E4                                                                 SUJ2       F1 A          Impossible                                                                          A    B          Impossible                                F2            due to same           due to same                               F3            dimensions            dimensions                                F4                                                                 SUJ2       G1 A          Impossible                                                                          A    B          Impossible                                G2            due to same           due to same                               G3            dimensions            dimensions                                G4                       B                                         SUJ2       H1 A          Impossible                                                                          A    B          Impossible                                H2            due to same           due to same                               H3            dimensions            dimensions                     SUJ2       I1 A          Impossible                                                                          A    B          Impossible                                I2            due to same           due to same                                             dimensions            dimensions                     Comparative                                                                         SUJ2 J1 250° C. Heated Element                                                            D  D  --   250° C. Heated                                                                    Dlement                                                                          D                           Examples   J2 500° C. Heated Element                                                            D  D  --   500° C. Heated                                                                    Dlement                                                                          D                                      J3 All D due to being impossible for forcing                       SUJ2       K1 A          Impossible                                                                          A    B          Impossible                     SUJ2       L1 A          Impossible                                                                          A    B          Impossible                                L2            due to same           due to same                                             dimensions            dimensions                     SUJ2       M1 D          D     A    D          B                                         M2 D          D     A    D          B                              SUJ2       N1 A          A     A    B          D                                         N2                  A                                              SUJ2       O1 B          D     --   D          D                                         O2                  --                                             __________________________________________________________________________

                  TABLE 2D                                                        ______________________________________                                        Steel               Hardness Tempering                                                                             Rapid Treat-                             Species      No.    (HRC)    Consistency                                                                           ment Time                                ______________________________________                                        Inventive                                                                             SUJ2     A1     HRC    A       A                                      Examples         A2     56-66                                                                  A3                                                                            A4                                                                            A5                                                                            A6                                                                   SUJ2     B1            A       A                                                       B2                                                                   SCR420   C1            A       A                                                       C2                                                                   SUS440C  D1            A       A                                                       D2                                                                   SUJ2     E1            A       A                                                       E2                                                                            E3                                                                            E4                                                                   SUJ2     F1            B       A                                                       F2            A                                                               F3                                                                            F4                    B                                              SUJ2     G1            B       A                                                       G2                                                                            G3            A                                                               G4                    B                                              SUJ2     H1            B       A                                                       H2                                                                            H3            A       C                                              SUJ2     I1            B       C                                                       I2            A       C                                      Comparative                                                                           SUJ2     J1     HRC    A       --                                     Examples         J2     56-66                                                            J3   All D due to being impossible                                                 for forcing                                                   SUJ2         K1     HRC      D       A                                        SUJ2         L1     56-66    D       A                                                     L2              D       B                                        SUJ2         M1              A       A                                                     M2                                                               SUJ2         N1     HRC      A       A                                                     N2     48-52                                                     SUJ2         O1     HRC      A       D                                                     O2     56-66                                                     ______________________________________                                    

Incidentally, the degree of working for a workpiece to be corrected bythe mold in these tests was calculated in accordance with the followingequation. ##EQU3##

The lower limit of the degree of working is the minimum degree ofworking that permits the correction of the deformation of a workpiece.If the degree of working is calculated based on the inner diameter of amold and is found to be less than the lower limit, then the workpiececannot be corrected by such a mold. On the other hand, the upper limitof the degree of working is the maximum degree of working that permitsthe forcing of a workpiece into a mold. If the degree of working iscalculated based on the inner diameter of a mold and is found to be morethan the upper limit, the workpiece cannot be forced into such a moldbecause the outer diameter of the workpiece (at room temperature) is toolarge for the inner diameter of the mold.

The upper limit of the degree of working δ is determined in thefollowing way.

Let it be assumed that the inner diameter d of a correcting mold is setconstant and that the maximum outer diameter of a workpiece permittingthe workpiece to be forced into the correcting mold before correctivetempering is D₀. Since equation (1) expressing the degree of working δcan be converted into ##EQU4##

Therefore, a curve defined by K₁ =K₁₀, K₂ =K₂₀ that are obtained bysubstituting D=D₀ into K₁, K₂ in equation (2) expresses δ_(max) (%)permitting the forcing of the workpiece into the mold. That is, theupper limit of the degree of working is defined by

    δ.sub.max =K.sub.10 +K.sub.20 T                      (3)

The outer diameter of specimen A6 was the limit that permits a specimento be forced into the mold in this invention. The upper limit δ of thedegree of working, i.e., δ_(max) (%) of equation (3) at maximumworkpiece temperatures of 250° C. and 500° C. can be calculated asfollows from Table 2A.

Since the linear expansion coefficient α of the steel grade SUJ2 is0.000012, ##EQU5## at 250° C. ##EQU6## at 500° C.

Further, K₁₀, K₂₀ are

    K.sub.10 =100×(47.20-47.0)/47.0≈0.43

    K.sub.20 =100×47.2×α/47.0 ≈100.4α

It may be noted that K₂₀ =1.21×10⁻³ when α=0.000012 is substituted.

K₁₀, K₂₀ were obtained on a test basis assuming that d=47.00 and thelike. Even taking into account the fact that the roundnesses of hardenedworkpieces are not consistent and the fact that the dimensions ofworkpieces and correcting molds are changed, K₁₀, K₂₀ can be defined asfollows.

    K.sub.10 =0.43±0.03

    K.sub.20 =(100.4±0.2)α

While differences between room temperatures and 0° C. and fluctuationsbrought about by the temperature in the inner diameter d of a correctingmold have been neglected in the aforementioned equations (1) to (3) andthe like, all these values are very small in view of the elasticdeformability of a workpiece and the like. Accordingly, it can be saidthat substantially reliable approximations can be provided by theaforementioned equations and the like.

It was verified that all the combinations of the outer diameters of theworkpieces and the inner diameters of the molds of the invention werewithin the range between the upper limit δ (=δ_(max)) and the lowerlimit δ (=0.20) and that some of Comparative Examples were out of thatrange (specimen J3 exceeding the upper limit δ, so that specimen J3cannot be forced into the mold, and specimens J1 and J2 were smallerthan the lower limit δ so that specimens J1 and J2 cannot be corrected).

FIG. 17 is a graph showing the results of the corrective tempering testmade to compare Inventive Examples A1 to A6 with Comparative ExamplesM1, M2 and N1, N2. The abscissa indicates the maximum workpiecetemperature, whereas the ordinate indicates the correcting degree ofdeformation and the workpiece surface hardness (HRC).

The correcting degree of deformation differs from every heating time andis improved as increasing maximum workpiece temperature. When themaximum workpiece temperature exceeds 500° C., the correcting degree ofdeformation is no longer improved. The passable lower limit is 60% inthe graph. A more preferable passable lower limit is 80%.

It is understood from these results that a maximum workpiece temperaturerange satisfying both a passable surface hardness limit of HRC 56 within30 seconds of the heating time and a passable degree of correction of60% or more is between 250° C. and 500° C.

FIG. 18 is a plot illustrating a relationship between the maximumworkpiece temperature and the dimension (outer diameter) standardizingrate from the test results of Inventive Examples A1 to A6 in Tables 2Ato 2C. The dimension standardizing rate increases as the dimensionaldispersion after tempering decreases, compared with the dimensionaldispersion before tempering. Therefore, the effect of correcting formaking the dimensions consistent are significant. It is understood fromthis graph that the effect of correcting are satisfactory within amaximum workpiece temperature range of from 250 to 500° C. and that theeffect of correcting are more satisfactory within a range of from 300 to500° C., or still more satisfactory within a range of from 380 to 500°C.

FIG. 19 is a plot illustrating a relationship between the degree ofworking and the surface roughness Ra (μm) of the deformed surfaces (theouter diameter surfaces in this case) corrected by the correcting moldfrom the test results of Inventive Examples A1 to A6 in Tables 2A and2C.

If the worst surface roughness permitting correction is set to Ra 0.2μm, the degree of working may be 0.2% or more. Inventive Examples A1 toA6 all satisfy this degree of working. Further, it is understood fromFIG. 19 that the degree of working must, more preferably, be 0.6% ormore (Ra 0.1 μm or less).

FIG. 20 is a plot illustrating a relationship between the degree ofworking (in abscissa) and the correcting degree of deformation from thetest results of Inventive Examples A1 to A6 and Comparative Examples J1and J2 in Tables 2A and 2B.

It is apparent also from these results that a correcting degree ofdeformation of 60% or more is obtained only when the degree of workingis 0.2% or more. In other words, it is apparent that the lower limit ofthe degree of working is 0.2%.

FIG. 21 is a plot illustrating a relationship between the maximumworkpiece temperature (in abscissa) and the degree of working from thetest results of Inventive Examples A1 to A6 and Comparative Examples J1to J3 in Tables 2A and 2C. In FIG. 21, the horizontal line indicating adegree of working of 0.2% is the lower limit of the degree of working,i.e., the minimum degree of working that permits correction of aworkpiece. Further, while the lines of the degrees of working plotted byeach of Inventive Examples A1 to A6 and Comparative Examples J1 to J3are presented in the graph, it was found out that the upper limit of thedegree of working, i.e., the maximum degree of working permitting aworkpiece W to be forced into a mold is defined by the line of thedegree of working of Inventive Example A6 in this embodiment asdescribed above. That is, the maximum degree of working is determined bythe curve obtained within the range of K₁₀ =0.43±0.03, and K₂₀=(100.4±0.2) α in equation (3).

That is, it is possible to increase the upper limit of the degree ofworking δ up to the value so defined as described above from the lowerlimit of the degree of working, 0.20%, within the maximum workpiecetemperature range of from 250 to 500° C. The shaded area in the middleof FIG. 21 is the maximum workpiece temperature and the degree ofworking range permitting the workpiece W to be forced into the moldwhile satisfying all such effects of the invention as a surface hardnessof HRC 56 or more, a correcting degree of deformation of 60% or more, adimension standardizing rate of 30% or more, and a corrected surfaceroughness of less than Ra 0.2 μm.

FIG. 22 shows how the shape of a workpiece W affects correctivetempering.

Differences ΔH_(v) (surface hardness dispersion) between the largestvalue and the smallest value in the surface hardness (H_(v)) of theworkpiece W and ratios of the contact surface area between the mold 22and the workpiece W to the volume V (S/V ratio) of the workpiece W wereplotted with the abscissa indicating the difference ΔH_(v) and with theordinate indicating the S/V ratio with respect to a plurality ofspecimens including Inventive and Comparative Examples denoted asrespective reference symbols in Table 2A. The mold temperature was setto different values from 55 to 500° C. using the mold temperatureadjuster 24. The workpieces W, which were the specimens, were heatedthrough conduction heating from the mold 22 and through inductionheating using the induction heating coil 23 either singly or incombination. It may be noted that the workpiece temperature was 400° C.and that the average surface hardness ranged from HRC 58 to 60. Theupper limit surface hardness dispersion ΔH_(v) is 30 or less, or morepreferably 16 or less. Further, a S/V ratio of 0.27 or more is requiredwhen induction heating is employed, and a S/V ratio of 0.1 or more isrequired when only conduction heating is employed.

As increasing the S/V ratio, the surface hardness dispersion ΔH_(v)decreases in a heating method based mainly on induction heating and thetreatment time is shortened. In this case, the mold temperature is low,so that the workpiece W can be set in the mold easily without thermalexpansion. In addition, the workpiece W is cooled rapidly and istherefore naturally dropped after the treatment. This dispenses with theuse of the extracting mold, which in turn contributes to increasing thelife of the mold.

On the other hand, as decreasing the S/V ratio, the chances foremploying a heating method based mainly on conduction increase, whichprovides the advantage of allowing large workpieces to be treated withless surface hardness dispersion. However, this heating method entails along treatment time and makes the setting of a workpiece in and takingthe workpiece out of a mold hard, which in turn makes it likely toreduce the life of the mold.

Another deformation correcting test will be described. Separately fromthe aforementioned test, a test was carried out by subjecting workpiecesW, which are the outer races of ball bearings, to a deformationcorrecting treatment in order to study a relationship between correctingenergy and mold temperature. This another deformation correcting testwill now be described.

FIGS. 23A and 23B are a diagram showing a mold of a corrective temperingapparatus used for the test.

The workpieces W used in the test were the outer races of ball bearings,whose designations are 6808, 6808, 6212, and 6312 made of SUJ2. A totalof four S/V ratios, i.e., the ratio of the volume (V mm³) to the contactsurface area (S mm²) were arranged. These four values are: 1, 0, 0.5,0.25, and 0.17 for the above designations, respectively.

These outer races (workpieces W) were kept at 840° C. for 20 minutes,thereafter dipped into an oil bath for hardening, and then inserted intoa split mold 50 (trisected) that had been heated to a predeterminedtemperature. The workpieces W were heated and pressured for apredetermined time by inserting the split mold 50 into an outer mold 51,and then taken out of the mold 50. Then, the roundnesses and hardnessesof these workpieces W were measured, and a comparison was then made withthe roundnesses of these workpieces W after hardening and before thepressuring and heating of the corrective tempering.

FIG. 24 shows the results of these measurements.

The abscissa indicates the mold temperature measured by the moldtemperature adjuster. The ordinate indicates the correcting energy. Thecorrecting energy is calculated by the equation: Mold temperature (°C.)×Pressuring/heating time (sec)×10₋₃. FIG. 24 shows that the workpieceW heating energy decreases as increasing S/V ratio, and also shows a S/Vratio of each workpiece W which can satisfy both a correcting degree ofdeformation of 60% or more and a surface hardness of HRC 56 or more.That is, the S/V ratio which allows satisfactory correcting results tobe obtained is from 0.17 to 1.0.

As described in the foregoing, the corrective tempering methods forrolling elements according to the invention are characterized in thatthe hardening deformation of a rolling element made of steel iscorrected by utilizing plasticity exhibited during the process oftempering the hard steel structure of the rolling element brought aboutby hardening. That is, the deformation correction is made by the methodof heating (tempering) the steel rolling element having been subjectedto hardening to 250 to 500° C. for a time not exceeding 6 minutes whileapplying pressure to the rolling element. As a result, the deformationof the rolling element can be corrected within a short period of time.In addition, not only the smallest hardness required for maintaining therolling fatigue life of the rolling element, which is HRC 56, can beensured, but also the amount of warpage or deformation of the outerdiameter or inner diameter of the rolling element required to ensureproduct accuracies can be kept within a prescribed value. The method inparticular involves induction heating for heating the rolling element,and corrects the deformation of the rolling element by treating therolling element for a time not exceeding 30 seconds at a degree ofworking ranging from 0.2 to the upper limit δ. Hence, according to thecorrective tempering methods of the invention, corrective tempering canbe completed within such an extremely short time as not exceeding 6minutes in order to achieve a correcting degree of deformation of 60 %or more for inner and outer diameter deformations, a dimensionstandardizing rate of 30% or more for the inner and outer diameters, acorrected surface roughness of less than Ra 0.2 μm, and a surfacehardness of HRC 56 or more. In addition, only a small number of toolsand molds are required for a mass production of rolling elements, whichis another advantage. Therefore, the invention is suitably applied to ahighly efficient mass production of long-life and highly accuraterolling elements such as the inner and outer races as well as cages ofrolling bearings. The corrective tempering apparatus for rollingelements according to the invention is characterized in that not onlythe induction heater is included as a heating source for heating ahardened rolling element to a desired temperature, but also the moldtemperature adjusting device for adjusting the temperature of a moldthat binds the rolling element is arranged. As a result of thisconstruction, the rolling element can be treated within such anextremely short time as not exceeding 30 seconds through directinduction heating. In addition, fitting dimensions between the mold andthe rolling element can be adjusted by freely controlling the moldtemperature using the mold temperature adjusting device. Therefore, therolling element can be set in and taken out of the mold easily as wellas quickly. Hence, the corrective tempering apparatus of the inventioncan complete corrective tempering within such an extremely short time asnot exceeding 30 seconds in order to achieve a correcting degree ofdeformation of 60% or more for inner and outer diameter deformations, adimension standardizing rate of 30% or more for the inner and outerdiameters, a corrected surface roughness of less than Ra 0.2 μm, and asurface hardness of HRC 56 or more. In addition, only a small number oftools and molds are required for a mass production of rolling elements,which is another advantage. Therefore, the invention is suitably appliedto a highly efficient mass production of long-life and highly accuraterolling elements such as the inner and outer races as well as cages ofrolling bearings.

What is claimed is:
 1. A method of correctively tempering a rollingelement selected from the group consisting of: a hardened cylindricalelement, a hardened annular element, and a hardened flat shape element,wherein the rolling element is made of steel selected from the groupconsisting of: carbon steel containing 0.5 wt % or more of carbon,bearing steel containing 0.5 wt % or more of carbon, and case hardeningsteel containing 0.5 wt % or less of carbon, the method comprising thesteps of:(a) inserting or pressuring the rolling element into a mold towork at least one surface of an inner diameter surface, an outerdiameter surface, and end faces of the rolling element; and (b) heatingthe rolling element for a heating time within six minutes required toreach a maximum rolling element temperature within 250 to 500° C. so asto correct a hardening deformation of the rolling element, wherein therolling element has a surface hardness of HRC 56 or more.
 2. The methodof claim 1, wherein step (b) is performed by conduction heating therolling element while a S/V ratio is set to 0.1 to 1.0, the S/V ratiobeing a ratio of a contact surface area (S) between the rolling elementand the mold to a volume (V) of the rolling element.
 3. The method ofclaim 1, wherein in the step (b) the rolling element is inductivelyheated for the heating time set to 30 seconds or less, and correctivelyworked to a degree of working which is a value ranging from 0.2% toδ_(max) defined by:

    δ.sub.max =K.sub.10 +K.sub.20 T

where K₁₀ =0.43±0.03, K₂₀ =(100.4±0.2)·α, T is the maximum rollingelement temperature, and α is the linear expansion coefficient.
 4. Themethod of claim 1, wherein step (b) is performed by induction heatingthe rolling element while a S/V ratio is set to 0.27 to 1.0, the S/Vratio being a ratio of a contact surface area (S) between the rollingelement and the mold to a volume (V) of the rolling element.
 5. Themethod of claim 1, wherein the rolling element has a correction degreeof deformation of 60% or more and has a dimension standardizing rate of30% or more after the deformation correction in said step (b).
 6. Themethod of claim 5, wherein the rolling element has a corrected surfaceroughness of Ra 0.2 μm or less.